1. Field of the Invention:
The present invention relates to dynamic motor control. More specifically, the present invention relates to the control of dynamic motors that are rotated between fixed angles and held stationary at those fixed angles for a specified period of time.
While the present invention is described herein with reference to a particular embodiment for an illustrative application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teaching provided herein will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art:
Various braking systems are known in the art. Such braking systems range from mechanical drum or disk brakes to the magnetic braking arrangements of three-phase induction motors.
In the control of dynamic motors it is desirable to maintain the position of a rotor at a specified location for a specified period of time. As is known in the art, a stepper motor uses electromagnetic detents to maintain a rotor at a desired location. Electrical energy must be added to cause the rotor to move past a magnetic hill. When the motor is de-energized, the rotor will settle and be maintained within a magnetic valley. A stepper motor must therefore be designed with these magnetic valleys located at the desired stopping positions. Thus one drawback of stepper motors is the requirement of a large number of detents in order to allow the stepper motor to be held at a large number of positions.
Another drawback of stepper motors is due to the correlation between retaining torque and magnetic friction. That is, the greater the restraining torque, the greater the magnitude of the magnetic hill and accordingly, the greater the force required to turn the rotor past the magnetic hill. Therefore if a large restraining torque is required to hold the rotor and its payload at a desired position, a large force will be required to turn the rotor from one stopping location to another.
A third drawback of stepper motors is due to the fact that as the motor settles in the magnetic valley, the motor shaft may vibrate before coming to rest at the desired location. This vibration could cause damage or excess wear to the motor or to a payload.
The micro-stepper motor offers an improvement over the stepper motor in that it allows a shaft to be held at an indefinite number of positions. The micro-stepper motor operates by applying electrically orthogonal signals to spatially orthogonal poles of a motor. By changing the ratio of magnitude of the signals applied, the motor can be moved to and held at any desired location.
In order to hold the rotor at a desired location, however, electrical energy must be applied constantly. As a result, the performance of micro-stepper motors has been limited by the requirement of a corresponding increase in applied electrical energy for greater restraining torques.
Accordingly, a need has been recognized in the art for a compact, lightweight, inexpensive braking system for dynamic motors with minimal restraining torque energy requirements.